Tough Hydrogels with Programmable and Complex Shape Deformations by Ion Dip‐Dyeing and Transfer Printing

Stimuli‐responsive hydrogels with high mechanical strength, programmable deformation, and simple preparation are essential for their practical applications. Here the preparation of tough hydrogels with programmable and complex shape deformations is reported. Janus hydrogels with different compositions and hydrophilic natures on the two surfaces are first prepared, and they exhibit reversible bending/unbending upon swelling/deswelling processes. More impressively, the deformation rate and extent of the hydrogels can further be easily controlled through an extremely simple and versatile ion dip‐dyeing (IDD) and/or ion transfer printing (ITP) method. By selectively printing proper patterns on 1D gel strips, 2D gel sheets and 3D gel structures, the transformations from 1D to 2D, 2D to 3D, and 3D to more complicated 3D shapes can be achieved after swelling the ion‐patterned hydrogels in water. The swelling‐deformable Janus and ion‐patterned hydrogels with high mechanical strengths and programmable deformations can find many practical applications, such as soft machines.     

4D Printing of Hydrogels: A Review

3D printing permits the construction of objects by layer‐by‐layer deposition of material, resulting in precise control of the dimensions and properties of complex printed structures. Although 3D printing fabricates inanimate objects, the emerging technology of 4D printing allows for animated structures that change their shape, function, or properties over time when exposed to specific external stimuli after fabrication. Among the materials used in 4D printing, hydrogels have attracted growing interest due to the availability of various smart hydrogels. The reversible shape‐morphing in 4D printed hydrogel structures is driven by a stress mismatch arising from the different swelling degrees in the parts of the structure upon application of a stimulus. This review provides the state‐of‐the‐art of 4D printing of hydrogels from the materials perspective. First, the main 3D printing technologies employed are briefly depicted, and, for each one, the required physico‐chemical properties of the precursor material. Then, the hydrogels that have been printed are described, including stimuli‐responsive hydrogels, non‐responsive hydrogels that are sensitive to solvent absorption/desorption, and multimaterial structures that are totally hydrogel‐based. Finally, the current and future applications of this technology are presented, and the requisites and avenues of improvement in terms of material properties are discussed.     

Direct 3D Printing of High Strength Biohybrid Gradient Hydrogel Scaffolds for Efficient Repair of Osteochondral Defect

The emerging 3D printing technique allows for tailoring hydrogel‐based soft structure tissue scaffolds for individualized therapy of osteochondral defects. However, the weak mechanical strength and uncontrollable swelling intrinsic to conventional hydrogels restrain their use as bioinks. Here, a high‐strength thermoresponsive supramolecular copolymer hydrogel is synthesized by one‐step copolymerization of dual hydrogen bonding monomers, N‐acryloyl glycinamide, and N‐[tris(hydroxymethyl)methyl] acrylamide. The obtained copolymer hydrogels demonstrate excellent mechanical properties—robust tensile strength (up to 0.41 MPa), large stretchability (up to 860%), and high compressive strength (up to 8.4 MPa). The rapid thermoreversible gel ? sol transition behavior makes this copolymer hydrogel suitable for direct 3D printing. Successful preparation of 3D‐printed biohybrid gradient hydrogel scaffolds is demonstrated with controllable 3D architecture, owing to shear thinning property which allows continuous extrusion through a needle and also immediate gelation of fluid upon deposition on the cooled substrate. Furthermore, this biohybrid gradient hydrogel scaffold printed with transforming growth factor beta 1 and β‐tricalciumphosphate on distinct layers facilitates the attachment, spreading, and chondrogenic and osteogenic differentiation of human bone marrow stem cells (hBMSCs) in vitro. The in vivo experiments reveal that the 3D‐printed biohybrid gradient hydrogel scaffolds significantly accelerate simultaneous regeneration of cartilage and subchondral bone in a rat model.     

3D Printing of Multifunctional Hydrogels

3D printing technology has been widely explored for the rapid design and fabrication of hydrogels, as required by complicated soft structures and devices. Here, a new 3D printing method is presented based on the rheology modifier of Carbomer for direct ink writing of various functional hydrogels. Carbomer is shown to be highly efficient in providing ideal rheological behaviors for multifunctional hydrogel inks, including double network hydrogels, magnetic hydrogels, temperature‐sensitive hydrogels, and biogels, with a low dosage (at least 0.5% w/v) recorded. Besides the excellent printing performance, mechanical behaviors, and biocompatibility, the 3D printed multifunctional hydrogels enable various soft devices, including loadable webs, soft robots, 4D printed leaves, and hydrogel Petri dishes. Moreover, with its unprecedented capability, the Carbomer‐based 3D printing method opens new avenues for bioprinting manufacturing and integrated hydrogel devices.     

Combining 3D Printing with Electrospinning for Rapid Response and Enhanced Designability of Hydrogel Actuators

Porous structures have emerged as a breakthrough of shape‐morphing hydrogels to achieve a rapid response. However, these porous actuators generally suffer from a lack of complexity and diversity in obtained 3D shapes. Herein, a simple yet versatile strategy is developed to generate shape‐morphing hydrogels with both fast deformation and enhanced designability in 3D shapes by combining two promising technologies: electrospinning and 3D printing. Elaborate patterns are printed on mesostructured stimuli‐responsive electrospun membranes, modulating in‐plane and interlayer internal stresses induced by swelling/shrinkage mismatch, and thus guiding morphing behaviors of electrospun membranes to adapt to changes of the environment. With this strategy, a series of fast deformed hydrogel actuators are constructed with various distinctive responsive behaviors, including reversible/irreversible formations of 3D structures, folding of 3D tubes, and formations of 3D structures with multi low‐energy states. It is worth noting that although poly(N‐isopropyl acrylamide) is chosen as the model system in the present research, our strategy is applicable to other stimuli‐responsive hydrogels, which enriches designs of rapid deformed hydrogel actuators.     

Transfer Printing of Cell Layers with an Anisotropic Extracellular Matrix Assembly using Cell‐Interactive and Thermosensitive Hydrogels

The structure of tissue plays a critical role in its function and therefore a great deal of attention has been focused on engineering native tissue‐like constructs for tissue engineering applications. Transfer printing of cell layers is a new technology that allows controlled transfer of cell layers cultured on smart substrates with defined shape and size onto tissue‐specific defect sites. Here, the temperature‐responsive swelling‐deswelling of the hydrogels with groove patterns and their versatile and simple use as a template to harvest cell layers with anisotropic extracellular matrix assembly is reported. The hydrogels with a cell‐interactive peptide and anisotropic groove patterns are obtained via enzymatic polymerization. The results show that the cell layer with patterns can be easily transferred to new substrates by lowering the temperature. In addition, multiple cell layers are stacked on the new substrate in a hierarchical manner and the cell layer is easily transplanted onto a subcutaneous region. These results indicate that the evaluated hydrogel can be used as a novel substrate for transfer printing of artificial tissue constructs with controlled structural integrity, which may hold potential to engineer tissue that can closely mimic native tissue architecture.     

Programmed Deformations of 3D‐Printed Tough Physical Hydrogels with High Response Speed and Large Output Force

Shape‐morphing hydrogels have emerging applications in biomedical devices, soft robotics, and so on. However, successful applications require a combination of excellent mechanical properties and fast responding speed, which are usually a trade‐off in hydrogel‐based devices. Here, a facile approach to fabricate 3D gel constructs by extrusion‐based printing of tough physical hydrogels, which show programmable deformations with high response speed and large output force, is described. Highly viscoelastic poly(acrylic acid‐co‐acrylamide) (P(AAc‐co‐AAm)) and poly(acrylic acid‐co‐N‐isopropyl acrylamide) (P(AAc‐co‐NIPAm)) solutions or their mixtures are printed into 3D constructs by using multiple nozzles, which are then transferred into FeCl₃ solution to gel the structures by forming robust carboxyl–Fe³⁺ coordination complexes. The printed gel fibers containing poly(N‐isopropyl acrylamide) segment exhibit considerable volume contraction in concentrated saline solution, whereas the P(AAc‐co‐AAm) ones do not contract. The mismatch in responsiveness of the gel fibers affords the integrated 3D gel constructs the shape‐morphing ability. Because of the small diameter of gel fibers, the printed gel structures deform and recover with a fast speed. A four‐armed gripper is designed to clamp plastic balls with considerable holding force, as large as 115 times the weight of the gripper. This strategy should be applicable to other tough hydrogels and broaden their applications.     

A Self-Thickening and Self-Strengthening Strategy for 3D Printing High-Strength and Antiswelling Supramolecular Polymer Hydrogels as Meniscus Substitutes

3D printing of high-strength and antiswelling hydrogel-based load-bearing soft tissue scaffolds with similar geometric shape to natural tissues remains a great challenge owing to insurmountable trade-off between strength and printability. Herein, capitalizing on the concentration-dependent H-bonding-strengthened mechanism of supramolecular poly(N-acryloyl glycinamide) (PNAGA) hydrogel, a self-thickening and self-strengthening strategy, that is, loading the concentrated NAGA monomer into the thermoreversible low-strength PNAGA hydrogel is proposed to directly 3D printing latently H-bonding-reinforced hydrogels. The low-strength PNAGA serves to thicken the concentrated NAGA monomer, affording an appropriate viscosity for thermal-assisted extrusion 3D printing of soft PNAGA hydrogels bearing NAGA monomer and initiator, which are further polymerized to eventually generate high-strength and antiswelling hydrogels, due to the reconstruction of strong H-bonding interactions from postcompensatory PNAGA. Diverse polymer hydrogels can be printed with self-thickened corresponding monomer inks. Further, the self-thickened high-strength PNAGA hydrogel is printed into a meniscus, which is implanted in rabbit's knee as a substitute with in vivo outcome showing an appealing ability to efficiently alleviate the cartilage surface wear. The self-thickening strategy is applicable to directly printing a variety of polymer-hydrogel-based tissue engineering scaffolds without sacrificing mechanical strength, thus circumventing problems of printing high-strength hydrogels and facilitating their application scope.     

Inkjet Printing of a Highly Loaded Palladium Ink for Integrated,Low‐Cost pH Sensors

An inkjet printing process for depositing palladium (Pd) thin films from a highly loaded ink (>14 wt%) is reported. The viscosity and surface tension of a Pd‐organic precursor solution is adjusted using toluene to form a printable and stable ink. A two‐step thermolysis process is developed to convert the printed ink to continuous and uniform Pd films with good adhesion to different substrates. Using only one printing pass, a low electrical resistivity of 2.6 μΩ m of the Pd film is obtained. To demonstrate the electrochemical pH sensing application, the surfaces of the printed Pd films are oxidized for ion‐to‐electron transduction and the underlying layer is left for electron conduction. Then, solid‐state reference electrodes are integrated beside the bifunctional Pd electrodes by inkjet printing. These potentiometric sensors have sensitivities of 60.6 ± 0.1 and 57 ± 0.6 mV pH^?1 on glass and polyimide substrates, and short response times of 11 and 6 s, respectively. Also, accurate pH values of real water samples are obtained by using the printed sensors with a low‐cost multimeter. These results indicate that the facile and cost‐effective inkjet printing and integration techniques may be applied in fabricating future electrochemical monitoring systems for environmental parameters and human health conditions.     

3D Electrophoresis‐Assisted Lithography (3DEAL): 3D Molecular Printing to Create Functional Patterns and Anisotropic Hydrogels

The ability to easily generate anisotropic hydrogel environments made from functional molecules with microscale resolution is an exciting possibility for the biomaterials community. This study reports a novel 3D electrophoresis‐assisted lithography (3DEAL) platform that combines elements from proteomics, biotechnology, and microfabrication to print well‐defined 3D molecular patterns within hydrogels. The potential of the 3DEAL platform is assessed by patterning immunoglobulin G, fibronectin, and elastin within nine widely used hydrogels and characterizing pattern depth, resolution, and aspect ratio. Furthermore, the technique's versatility is demonstrated by fabricating complex patterns including parallel and perpendicular columns, curved lines, gradients of molecular composition, and patterns of multiple proteins ranging from tens of micrometers to centimeters in size and depth. The functionality of the printed molecules is assessed by culturing NIH‐3T3 cells on a fibronectin‐patterned polyacrylamide‐collagen hydrogel and selectively supporting cell growth. 3DEAL is a simple, accessible, and versatile hydrogel‐patterning platform based on controlled molecular printing that may enable the development of tunable, chemically anisotropic, and hierarchical 3D environments.     

Complex 3D‐Printed Microchannels within Cell‐Degradable Hydrogels

3D‐printing is emerging as a technology to introduce microchannels into hydrogels, for the perfusion of engineered constructs. Although numerous techniques have been developed, new techniques are still needed to obtain the complex geometries of blood vessels and with materials that permit desired cellular responses. Here, a printing process where a shear‐thinning and self‐healing hydrogel “ink” is injected directly into a “support” hydrogel with similar properties is reported. The support hydrogel is further engineered to undergo stabilization through a thiol‐ene reaction, permitting (i) the washing of the ink to produce microchannels and (ii) tunable properties depending on the crosslinker design. When adhesive peptides are included in the support hydrogel, endothelial cells form confluent monolayers within the channels, across a range of printed configurations (e.g., straight, stenosis, spiral). When protease‐degradable crosslinkers are used for the support hydrogel and gradients of angiogenic factors are introduced, endothelial cells sprout into the support hydrogel in the direction of the gradient. This printing approach is used to investigate the influence of channel curvature on angiogenic sprouting and increased sprouting is observed at curved locations. Ultimately, this technique can be used for a range of biomedical applications, from engineering vascularized tissue constructs to modeling in vitro cultures.     

Wet‐Responsive,Reconfigurable, and Biocompatible Hydrogel Adhesive Films for Transfer Printing of Nanomembranes

This study presents a wet‐responsive and biocompatible smart hydrogel adhesive that exhibits switchable and controllable adhesions on demand for the simple and efficient transfer printing of nanomembranes. The prepared hydrogel adhesives show adhesion strength as high as ≈191 kPa with the aid of nano‐ or microstructure arrays on the surface in the dry state. When in contact with water, the nano/microscopic and macroscopic shape reconfigurations of the hydrogel adhesive occur, which turns off the adhesion (≈0.30 kPa) with an extremely high adhesion switching ratio (>640). The superior adhesion behaviors of the hydrogels are maintained over repeating cycles of hydration and dehydration, indicating their ability to be used repeatedly. The adhesives are made of a biocompatible hydrogel and their adhesion on/off can be controlled with water, making the adhesives compatible with various materials and surfaces, including biological substrates. Based on these smart adhesion capabilities, diverse metallic and semiconducting nanomembranes can be transferred from donor substrates to either rigid or flexible surfaces including biological tissues in a reproducible and robust fashion. Transfer printing of a nanoscale crack sensor onto a bovine eye further demonstrates the potential of the reconfigurable hydrogel adhesive for use as a stimuli‐responsive, smart, and versatile functional adhesive for nanotransfer printing.     

Self‐Organization of Inkjet‐Printed Organic Semiconductor Films Prepared in Inkjet‐Etched Microwells

The high‐precision deposition of highly crystalline organic semiconductors by inkjet printing is important for the production of printed organic transistors. Herein, a facile nonconventional lithographic patterning technique is developed for fabricating banks with microwell structures by inkjet printing solvent droplets onto a polymer layer, thereby locally dissolving the polymer to form microwells. The semiconductor ink is then inkjet‐printed into the microwells. In addition to confining the inkjet‐printed organic semiconductor droplets, the microwells provide a platform onto which organic semiconductor molecules crystallize during solvent evaporation. When printed onto the hydrophilic microwells, the inkjet‐printed 6,13‐bis(triisopropylsilylethynyl) pentacene (TIPS_PEN) molecules undergo self‐organization to form highly ordered crystalline structures as a result of contact line pinning at the top corner of the bank and the outward hydrodynamic flow within the drying droplet. By contrast, small crystallites form with relatively poor molecular ordering in the hydrophobic microwells as a result of depinning of the contact line along the walls of the microwells. Because pinning in the hydrophilic microwells occurred at the top corner of the bank, treating the surfaces of the dielectric layer with a hydrophobic organic layer does not disturb the formation of the highly ordered TIPS_PEN crystals. Transistors fabricated on the hydrophilic microwells and the hydrophobic dielectric layer exhibit the best electrical properties, which is explained by the solvent evaporation and crystallization characteristics of the organic semiconductor droplets in the microwell. These results indicate that this technique is suitable for patterning organic semiconductor deposits on large‐area flexible substrates for the direct‐write fabrication of high‐performance organic transistors.     

Gradient Porous Elastic Hydrogels with Shape‐Memory Property and Anisotropic Responses for Programmable Locomotion

Programmable locomotion of responsive hydrogels has gained increasing attention for potential applications in soft robotics, microfluidic components, actuators, and artificial muscle. Modulation of hydrogel pore structures is essential for tailoring their mechanical strength, response speeds, and motion behaviors. Conventional methods forming hydrogels with homogeneous or stepwise‐distributed pore structures are limited by the required compromise to simultaneously optimize these aspects. Here, a heterobifunctional crosslinker enabled hydrothermal process is introduced to synthesize responsive hydrogels with well‐defined gradient pore construction. According to gradient porosity controls, the hydrogels simultaneously exhibit rapid responses to external stimuli, high elasticity/compressibility, and programmable locomotion capability. By incorporating polypyrrole nanoparticles as photothermal transducers, photo/thermal responsive composite hydrogels are formed to enable programmable control of locomotion such as bending, curving, twisting, and octopus‐like swimming under near‐infrared laser stimulation. The tunable pore structures, mechanical properties, and locomotion of this new class of materials make these gradient porous hydrogels potentially suitable for a variety of applications.     

Hydrogel−Solid Hybrid Materials for Biomedical Applications Enabled by Surface‐Embedded Radicals

Combinations of hydrogels and solids provide high level functionality for devices such as tissue engineering scaffolds and soft machines. However, the weak bonding between hydrogels and solids hampers functionality. Here, a versatile strategy to develop mechanically robust solid?hydrogel hybrid materials using surface embedded radicals generated through plasma immersion ion implantation (PIII) of polymeric surfaces is reported. Evidence is provided that the reactive radicals play a dual role: inducing surface‐initiated, spontaneous polymerization of hydrogels; and binding the hydrogels to the surfaces. Acrylamide and silk hydrogels are formed and covalently attached through spontaneous reactions with the radicals on PIII activated polymer surfaces without cross‐linking agents or initiators. The hydrogel amount increases with incubation time, monomer concentration, and temperature. Stability tests indicate that 95% of the hydrogel is retained even after 4 months in PBS solution. T‐peel tests show that failure occurs at the tape?hydrogel interface and the hydrogel‐PIII‐treated PTFE interfacial adhesion strength is over 300 N m^?1. Cell assays show no adhesion to the as‐synthesized hydrogels; however, hydrogels synthesized with fibronectin enable cell adhesion and spreading. These results show that polymers functionalized with surface‐embedded radicals provide excellent solid platforms for the generation of robust solid?hydrogel hybrid structures for biomedical applications.     

Ultrafast Fabrication of Gradient Nanoporous All‐Polysaccharide Films as Strong,Superfast, and Multiresponsive Actuators

The design of smart hydrogel actuators fully constructed from natural polymers for assessing the biomedical applications is important but challenging. Herein, an extremely simple, green, and ultrafast strategy is presented for preparing robust gradient all‐polysaccharide polyelectrolyte complex hydrogel actuators. Driven by diffusing of low molecular weight chitosan into high molecular weight sodium alginate solution, a nanoporous, ultrastrong, and gradient chitosan/sodium alginate complex hydrogel film with adjustable thickness can be directly generated on the interface of two solutions within minutes. The as‐prepared film can provide superfast temperature, ionic strength, and pH‐triggered programmable deformations, and perform a distinct sequential double folding behavior due to the competitive effect between complexed and noncomplexed segments of polyelectrolyte. Besides, patterning Ca²⁺ to locally crosslink sodium alginate in the film enables various more complex shape transformations. This green and simple diffusion‐driven strategy provides significant guidance for fabricating bio‐friendly actuators with applications in drug delivery, tissue engineering, soft robotics, and active implants.     

Inkjet‐Printed Single‐Droplet Organic Transistors Based on Semiconductor Nanowires Embedded in Insulating Polymers

Fabrication of organic field‐effect transistors (OFETs) using a high‐throughput printing process has garnered tremendous interest for realizing low‐cost and large‐area flexible electronic devices. Printing of organic semiconductors for active layer of transistor is one of the most critical steps for achieving this goal. The charge carrier transport behavior in this layer, dictated by the crystalline microstructure and molecular orientations of the organic semiconductor, determines the transistor performance. Here, it is demonstrated that an inkjet‐printed single‐droplet of a semiconducting/insulating polymer blend holds substantial promise as a means for implementing direct‐write fabrication of organic transistors. Control of the solubility of the semiconducting component in a blend solution can yield an inkjet‐printed single‐droplet blend film characterized by a semiconductor nanowire network embedded in an insulating polymer matrix. The inkjet‐printed blend films having this unique structure provide effective pathways for charge carrier transport through semiconductor nanowires, as well as significantly improve the on‐off current ratio and the environmental stability of the printed transistors.     

4D Printing of Multi-Stimuli Responsive Protein-Based Hydrogels for Autonomous Shape Transformations

Stimuli responsive hydrogels that can change shape in response to applied external stimuli are appealing for soft robotics, biomedical devices, drug delivery, and actuators. However, existing 3D printed shape morphing materials are non-biodegradable, which limits their use in biomedical applications. Here, 3D printed protein-based hydrogels are developed and applied for programmable structural changes under the action of temperature, pH, or an enzyme. Key to the success of this strategy is the use of methacrylated bovine serum albumin (MA–BSA) as a biodegradable building block to Pickering emulsion gels in the presence of N-isopropylacrylamide or 2-dimethylaminoethyl methacrylate. These shear-thinning gels are ideal for direct ink write (DIW) 3D printing of multi-layered stimuli-responsive hydrogels. While poly(N-isopropylacrylamide) and poly(dimethylaminoethyl methacrylate) introduce temperature and pH-responsive properties into the printed objects, a unique feature of this strategy is an enzyme-triggered shape transformation based on the degradation of the bovine serum albumin network. To highlight this technique, protein-based hydrogels that reversibly change shape based on environmental temperature and pH are fabricated, and irreversibly altered by enzymatic degradation, which demonstrates the complexity that can be introduced into 4D printed systems.     

Highly Customizable All Solution–Processed Polymer Light Emitting Diodes with Inkjet Printed Ag and Transfer Printed Conductive Polymer Electrodes

Inkjet and transfer printing processes are combined to easily form patterned poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films as top anodes of all solution–processed inverted polymer light emitting diodes (PLEDs) on rigid glass and flexible plastic substrates. An adhesive PEDOT:PSS ink is formulated and fully customizable patterns are obtained using the inkjet printing process. In order to transfer the patterned PEDOT:PSS films, adhesion properties at interfaces during multistep transfer printing processes are carefully adjusted. The transferred PEDOT:PSS film on the plastic substrates shows not only a sheet resistance of 260.6 Ω/□ and a transmittance of 92.1% at 550 nm wavelength but also excellent mechanical flexibility. The PLEDs with spin‐coated functional layers sandwiched between the transferred PEDOT:PSS top anodes and inkjet‐printed Ag bottom cathodes are fabricated. The fabricated PLEDs on the plastic substrates show a high current efficiency of 10.4 cd A^?1 and high mechanical stability. It is noted that because both Ag and PEDOT:PSS electrodes can be patterned with a high degree of freedom via the inkjet printing process, highly customizable PLEDs with various pattern sizes and shapes are demonstrated on the glass and plastic substrates. Finally, with all solution process, a 5 × 7 passive matrix PLED array is demonstrated.     

Inherent Photodegradability of Polymethacrylate Hydrogels: Straightforward Access to Biocompatible Soft Microstructures

Hydrogels are important functional materials useful for 3D cell culture, tissue engineering, 3D printing, drug delivery, sensors, or soft robotics. The ability to shape hydrogels into defined 3D structures, patterns, or particles is crucial for biomedical applications. Here, the rapid photodegradability of commonly used polymethacrylate hydrogels is demonstrated without the need to incorporate additional photolabile functionalities. Hydrogel degradation depths are quantified with respect to the irradiation time, light intensity, and chemical composition. It can be shown that these parameters can be utilized to control the photodegradation behavior of polymethacrylate hydrogels. The photodegradation kinetics, the change in mechanical properties of polymethacrylate hydrogels upon UV irradiation, as well as the photodegradation products are investigated. This approach is then exploited for microstructuring and patterning of hydrogels including hydrogel gradients as well as for the formation of hydrogel particles and hydrogel arrays of well‐defined shapes. Cell repellent but biocompatible hydrogel microwells are fabricated using this method and used to form arrays of cell spheroids. As this method is based on readily available and commonly used methacrylates and can be conducted using cheap UV light sources, it has vast potential to be applied by laboratories with various backgrounds and for diverse applications.